News From NCAR: Cool Climate, Hot Solar Research

Date:

December 16, 1999

Source:

National Center For Atmospheric Research (NCAR)

Summary:

This tip sheet highlights breakthroughs in understanding the cooling
effect of agriculture, the link between El Ninos and volcanoes, and the
length and strength of solar cycles. Presentations of these results
from the National Center for Atmospheric Research are part of the fall
meeting of the American Geophysical Union.

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FULL STORY

BOULDER -- New results from the National Center for Atmospheric Researchwith implications for climate and space weather are being presented thisweek during the fall meeting of the American Geophysical Union in SanFrancisco. Three presentations are highlighted below. NCAR's primarysponsor is the National Science Foundation.

--Agriculture Keeps the United States Cooler--

Converting forest to cropland in the central United States has cooleddaily maximum temperatures for deforested areas by 2 to 3 degreesCelsius (4 to 5 degrees Fahrenheit), according to an NCAR land-surfacemodel and a new analysis of observations.

Gordon Bonan analyzed daily maximum and minimum temperatures over thepast decade in the Midwest, which is primarily cropland, and comparedthem with the same information for the Northeast, which is primarilyforested. The analysis provides support for Bonan's model predictions:croplands reduce daily maximum temperatures, especially in summer andautumn, and the maximum temperatures are reduced more than dailyminimums, resulting in a decrease in the diurnal (24-hour) temperaturerange in the Midwest.

"You would expect diurnal temperature ranges to decrease as you go fromthe Midwest to the East," says Bonan, because clouds, which keepnighttime temperatures warmer, are heavier in the East. "But theobservations show the opposite, and that's consistent with what themodel suggests."

Bonan's findings suggest that human-induced changes other thangreenhouse emissions are affecting the climate. Some climate-changecritics have found reassurance in cooler temperatures over the UnitedStates. However, Bonan says, "When people say that temperatures in theUnited States have not warmed, we have to look at all the reasons whythey haven't warmed."

Large volcanic eruptions in the tropics over the past 400 years haveoften been followed by an El Nino pattern around the globe, according toa new climate data analysis at NCAR.

Paleoclimate student Caspar Ammann used local eruption observations andice-core data to track the years of major volcanic eruptions in thetropics. He then used a data set based on tree rings and coral growthlayers to arrive at a global picture of the earth's climate thefollowing year. He found a tendency for major tropical eruptions to befollowed by an El Nino-like pattern of temperature changes in thetropical waters of the eastern Pacific, Alaska, the southeastern UnitedStates, the tropical Atlantic, the Indian Ocean, and the westernPacific. He defined major eruptions as those powerful enough to ejectsulfates into the stratosphere, where they would remain for severalyears.

"It appears that a major volcanic eruption in the tropics can somehowtweak the normal interannual cycle to favor an early onset of El Nino,"says Ammann, "but we don't understand the mechanism." El Ninos tend torecur naturally every three to seven years. The next step is to modelthe climate over the past centuries using the volcanic data to see ifthe relationship is reproduced, then investigate why it occurs.

New computer models of the Sun's dynamics are helping to explain thelength and strength of the solar cycle. The NCAR modeling effort couldhelp provide a stronger physical basis for predicting the intensity ofupcoming solar maxima, such as the one expected this winter.

The Sun's activity tends to peak about every 11 years, but some cyclesare separated by as little as 9 years or as much as 14. Stronger cyclestend to be shorter than average ones, and weak cycles longer thanaverage. Most nonlinear dynamo models of the solar cycle have beenunable to explain this behavior until now. NCAR researchers MausumiDikpati and Paul Charbonneau believe they may have found the cause:variations in the meridional (north-south or south-north) flow of plasmaacross the Sun.

The meridional flow is typically quite weak, but over time it carrieslarge amounts of magnetic field poleward and helps recharge the solardynamo for its next activity peak. In a model that captures solarmagnetic field transport, Dikpati and Charbonneau have argued that themeridional flow can be easily disrupted by the intense vertical motionsof convection (parcels of heat rising from the Sun's surface). Thisconvection is naturally variable, which helps explain why its disruptionof the flow varies from cycle to cycle. Yet the amount of plasmatransported by the flow over the course of a cycle remains relativelyconstant; thus, at times when the flow is disrupted more than usual, theresulting solar cycle is weaker but lasts longer. "It's been said thatthe Sun has a clock and the clock is always adjusted," says Dikpati.

NCAR is managed by the University Corporation for Atmospheric Research,a consortium of more than 60 universities offering Ph.D.s in atmosphericand related sciences.

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